Sometimes the fieldlist is corrupted.
To correct this, you need to reset the list., e.g. like this:
select any file.
from the toolbar select "Actions (quick)"
Select the action type "Replace"
click on the button with the right-arrow and select "Reset field list" from the menu.
Cancel the action.
Hopefully, you see all fields again
Edit: no, you are right, resetting does not help. Sorry.
That seems to be working nicely.
I must say I wouldn't expect such a 'threshold' to make use of a standard id3v2.4 tag in MP3Tag, but that is probably saying more about my lack of understanding of the matter. (and your great program)